Introduction

Pests are organisms that interfere with the goals of humans. Plants that interfere with human activities are typically referred to as weeds. Such plants are unique as a category of pest as they are ecologically producers, in contrast with organisms in all other pest categories, which are consumers. Weeds are thus in a different ecological position from all other categories of pests, resulting in differences in control tactics, and making weeds and their management likely to interact with all other categories of pests.

Weeds, like all plants, are not mobile. Therefore weeds cannot “search”, in the sense of moving to a new location, for resources that they need. Lack of mobility also means that weeds cannot move to escape from organisms that would feed on them. These two differences between weeds and organisms in all animal pest categories mean that the approaches to weed management, and their response to such management, are often quite different from those for animal pests.

Most animal pests and aerial pathogens are not equally devastating every year. Their population dynamics lead to outbreak years when losses can be very high, and other years when populations remain low and losses are small. Weeds, on the other hand, like most other soil-borne pests, typically occur in damaging numbers every year once a site is infested, necessitating control action every year in order to achieve the crop yields desired by humans.